Optical quantizing unit and optical A/D converter

ABSTRACT

An optical quantizing unit includes an optical divider dividing 1 st  optical pulses to be quantized and sending the divided 1 st  optical pulses into a plurality of paths; a plurality of optical filters passing with different transmittances the divided 1 st  optical pulses; and an optical threshold filter sequentially receiving the 1 st  optical pulses, and sending 2 nd  optical pulses when light intensities of the 1 st  optical pulses are above a preset threshold value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of the priorityfrom prior Japanese Patent Application 2006-004214 filed on Jan. 11,2006, the entire contents of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical quantizing unit and an optical A/Dconverter, and more particularly relates to an optical quantizing unitwhich produces optical pulses whose quantities are proportional tointensities of input optical pulses, and an optical A/D converterprovided with the optical quantizing unit.

2. Description of the Related Art

U.S. Pat. No. 4,712,089 discloses an optical quantizing circuit whichproduces optical pulses in proportion to the intensity of opticalpulses.

In Reference 1, input optical pulses to be quantized are divided inaccordance with the number of quantizing levels, and the optical pulsesto be quantized are input into a plurality of optical threshold filtershaving different threshold values.

The optical threshold filters compare the optical pulses to be quantizedwith threshold values, and output optical pulses when the optical pulsesto be quantized have light intensities above the threshold values. Thenumber of the outputted optical pulses is proportional to intensities ofoptical pulses to be quantized.

However, since the foregoing optical threshold filters are large, theoptical quantizing unit inevitably become bulky as a whole. Further,when a plurality of optical threshold filters are provided, theapplication efficiency of light will be lowered, and intensities of theinput optical pulses to be quantized will be increased. Therefore, it isdifficult to fabricate an economical optical quantizing unit.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the embodiment of the invention, there isprovided an optical quantizing unit which includes an optical dividerdividing 1^(st) optical pulses to be quantized and sending the divided1^(st) optical pulses to a plurality of routes; a plurality of opticalfilters passing with different transmittances the divided 1^(st) opticalpulses; and an optical threshold filter sequentially receiving the1^(st) optical pulses, and sending 2^(nd) optical pulses when lightintensities of the 1^(st) optical pulses are above a preset thresholdvalue.

In accordance with a second aspect, there is provided an optical A/Dconverter which includes an optical sampling unit which samples opticalanalog signals; an optical quantizing unit which quantizes the sampledoptical analog signals and outputs the quantized optical pulses; and abinary converter which performs binary conversion of the quantizedoptical pulses. The optical quantizing unit is constituted by an opticaldivider which divides 1^(st) optical pulses to be quantized andtransmits the divided 1^(st) optical pulses to a plurality of paths; aplurality of optical filters which transmit with differenttransmittances the divided 1^(st) optical pulses; and an opticalthreshold filter which sequentially receives the 1^(st) optical pulsesfrom the optical filters, and outputs the quantized 2^(nd) opticalpulses when light intensities of the 1^(st) optical pulses are above apreset threshold value.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of an optical A/D converter according to afirst embodiment of the invention.

FIG. 2A, FIG. 2B, FIG. 2C and FIG. 2D show waveforms which are processedby the optical A/D converter of FIG. 1.

FIG. 3 is a block diagram of an optical quantizing unit in the opticalA/D converter of FIG. 1.

FIG. 4 is a graph showing the relationship between the intensities ofinput pulses and the number of output pulses in the optical quantizingunit of FIG. 3.

FIG. 5 is a block diagram of an optical threshold filter of the opticalquantizing unit of FIG. 3.

FIG. 6 schematically shows an optical threshold filter using bistablesemiconductor lasers according to a further embodiment of the invention.

FIG. 7 schematically shows an optical threshold filter using a nonlinearetalon.

FIG. 8 schematically shows an example of an optical quantizing unitwhich is integrated using a optical waveguide.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

Referring to FIG. 1, an optical A/D (Analog/Digital) converter 10includes an optical sampling unit 11, an optical quantizing unit 12, andan optical binary counter 13. The optical sampling unit 11 sequentiallysamples an optical analog signal S10 (see FIG. 2A), and converts thesignal S10 into a string of optical pulses P10 (P10 a, P10 b, P10 c, . .. , shown in FIG. 2B) to be quantized. The optical quantizing unit 12sequentially receives the optical pulses P10 to be quantized, andquantizes them into a string of quantized optical pulses P12 (P12 a, P12b, P12 c, . . . , shown in FIG. 2C). The optical binary counter 13sequentially receives the quantized optical pulses P12 from the opticalquantizing unit 12, and converts them into coded optical binary pulsesP13 (see FIG. 2D).

The optical sampling unit 11 may operate on four-wave-mixing, passes anoptical analog signal having a frequency ω2 and sampling pulses having afrequency ω1 through a nonlinear medium (e.g., a dispersion shiftedfiber or the like), and produces a string of analog pulses having afrequency of 2ω1-ω2 (whose amplitude is proportional to a signal levelof a sampled optical analog signal). By the way, the frequency ω, awavelength λ and a velocity c of light are expressed by ω=2πc/λ. Theoptical sampling unit 11 is not always required to operate on thefour-wave-mixing.

The optical quantizing unit 12 (shown in FIG. 3) is constituted by anoptical divider 21, a plurality of delay lines 22 a to 22 e, a pluralityof optical filters 23 a to 23 e, an optical synthesizer 24, and anoptical threshold filter 25. The optical divider 21 divides the opticalpulses P10 (P10 a, P10 b, P10 c, . . . ) to be quantized and direct themto a plurality of routes. The delay lines 22 a to 22 e delay the dividedoptical pulses P10 ₁ to P10 ₅ with different delay times. The opticalfilters 23 a to 23 e pass the optical pulses P23 a to P23 e withdifferent transmittances. The optical synthesizer 24 synthesizes theoptical pulses P23 a to P23 e. The optical threshold filter 25 sendsoutput pulses P25 (quantized optical pulses P12) when the intensities ofoptical pulses P23 a to P23 e are above the threshold P_(th).

In the optical quantizing unit 12, the optical divider 21 has a starcoupler which may be a 1×5 star coupler of fiber or waveguide type. Theoptical divider 21 is oriented in order to divide one input into fiveoutputs. Specifically, the optical divider 21 divides the optical pulsesP10 (P10 a, P10 b, P10 c, P10 d, . . . ) to be quantized into the numberof pulses in accordance with quantizing levels in the quantizing unit12. It is assumed that three optical pulses P10 a, P10 b and P10 c aresequentially received. For instance, the optical pulse P10 a is dividedinto five levels (however, the number of quantizing levels is not alwayslimited to five). The optical pulses P10 b and P10 c are similarlydivided into the number of levels in accordance with the quantizinglevels in the quantizing unit 12.

Thereafter, the optical divider 21 sends the divided optical pulses (P10₁ to P10 ₅) to a delay line array 22. The delay line array 22 includes aplurality of delay lines 22 a to 22 e whose waveguides have differentlengths. In other words, the divided optical pulses P10 ₁ to P10 ₅ aresent to the delay lines 22 a to 22 e, respectively. The waveguide of thedelay line 22 a is shortest, while the waveguides of the delay lines 22b, 22 c, 22 d and 22 e become longer in sequence. This means that adelay time is shortest in the delay line 22 a while the delay times ofthe delay lines 22 b, 22 c, 22 d and 22 e become longer in sequence. InFIG. 3, “τ” denotes delay times.

The optical pulses P10 ₁, P10 ₂, P10 ₃, P10 ₄ and P10 ₅ which have beendelayed in the delay line array 22 are transmitted to the opticalfilters 23 a to 23 e of the optical filter array 23. The optical filters23 a to 23 e pass the received optical pulses with differenttransmittances. In short, the light transmittances of the opticalfilters 23 a to 23 e are selected in order to satisfy the followingformula, where “t₁” denotes the largest transmittance, and “t₂” denotesa second largest transmittance.

$\frac{1}{t_{i + 2}} = {\frac{2}{t_{i + 1}} - \frac{1}{t_{i}}}$

Therefore, it is possible to adjust the light intensities of the opticalpulses P10 ₁ to P10 ₅ in the linear shape. For instance, when t₁=1 andt₂=½, t₃ is ⅓, t₄ is ¼ and t₅ is ⅕. The optical filter 23 a is placed atan output side of the delay line 22 a whose delay time is shortest, andoutputs the input optical pulse without reducing the light intensity.The optical filter 23 b is placed at an output side of the delay line 22b whose delay time is secondly shortest, and outputs the input opticalpulse by reducing the light intensity to half. The optical filter 23 cis placed at an output side of the delay line 22 c whose delay time isthirdly shortest, and outputs the input optical pulse by reducing thelight intensity to one third. The optical filter 23 d is placed at anoutput side of the delay line 22 d whose delay time is fourthlyshortest, and outputs the received optical pulse by reducing the lightintensity to one fourth. The optical filter 23 e is placed at an outputside of the delay line 22 e whose delay time is longest, and outputs theinput optical pulse by reducing the light intensity to one fifth. Theoptical filter 23 a passes the optical pulses without reducing the lightintensity, which is the same as in a case where there is no opticalfilter. Therefore, the optical filter 23 a may be omitted.

The optical pulse P23 a having the largest light intensity istransmitted first of all. Thereafter, the optical pulses P23 b, P23 c,P23 d and P23 e whose light intensities are sequentially reduced aretransmitted in succession. As for t_(i) and t_(i+1), it is not alwaysrequired that t₁ is 1 and t₂ is ½, but they may be 1.0 and 0.4,respectively, or may be any values.

FIG. 4 shows the relationship between intensities of the input pulsesand quantity of the output pulses when t₁ is 1 and t₂ is 0.5, and athreshold value P_(th) (to be described later) of the optical thresholdfilter 25 is 1.

The optical synthesizer 24 is placed downstream of the optical filterarray 23, and synthesizes outputs of the optical filters 23 a to 23 e.The optical synthesizer 24 is provided with a star coupler, which is ofa fiber or waveguide type 1×5 star coupler, and is oriented in order tosynthesize the five inputs into one output. Specifically, the opticalsynthesizer 24 synthesizes the outputs of the optical filters 23 a to 23e, and sequentially sends the optical pulses P23 a to P23 e to theoptical threshold filter 25 at specified timings. The specified timingsdepend upon the delay times of the delay lines 22 a to 22 e of the delayline array 22.

As described above, the optical divider 21 sequentially divides theoptical pulses P10 (P10 a, P10 b, P10 c, . . . ); the divided opticalpulses P10 are transmitted to the optical filters 23 a to 23 e via thedelay lines 22 a to 22 e; the optical filters 23 a to 23 e pass thedivided optical pulses P10 with the different transmittances to producethe optical pulses P23 a to P23 e having the foregoing lightintensities; and the optical synthesizer 24 periodically andsequentially transmits the optical pulses P23 a to P23 e to the opticalthreshold filter 25 at different timings.

The optical threshold filter 25 compares the received optical pulses P23a to P23 e with the preset threshold value P_(th), and transmits anoutput pulse P25 for an optical pulse whose light intensity is above thepreset threshold value P_(th). The output pulse P25 serves as thequantized optical pulse P12 shown in FIG. 1.

Referring to FIG. 5, in the optical threshold filter 25, a first laseroscillator 30 a includes a first optical amplifier 31 a and opticalfilters 32 a, 33 a. A first optical amplifier 31 a is placed betweenoptical filters 32 a and 33 a. A second laser oscillator 30 b includes asecond optical amplifier 31 b and optical filters 32 b, 33 b. A secondoptical amplifier 31 b is placed between optical filters 32 b and 33 b.

The optical amplifiers 31 a and 31 b are preferably erbium-doped fiberamplifiers or semiconductor optical amplifiers (SOA). The opticalthreshold filter 25 is constituted by the laser oscillators, and is veryrobust since it does not operate in response to optical phases.

The optical filters 32 a, 33 a, 32 b and 33 b include FBGs (Fiber BraggGratings). Each FBG is a diffraction grating whose flexibility variesperiodically, and is placed at a core of an optical fiber. These opticalfilters reflect only lights having predetermined wavelengths inaccordance with cycles of the diffraction gratings and flexibilities ofthe optical fibers, but pass the remaining lights.

In this embodiment, the optical filters 32 a and 33 a of the first laseroscillator 30 a are constituted by FBGs which reflect lights havingwavelengths slightly different from the wavelength of the optical pulsesP10 to be quantized. This enables the first laser oscillator 30 a tooscillate in response to the lights having the preset wavelengths.

In contrast to the first laser oscillator 30 a, the optical filters 32 band 33 b of the second laser oscillator 30 b are constituted by FBGswhich reflect lights having wavelengths slightly different from those ofthe optical pulses P10 to be quantized and reflected wavelengths ofoptical filters 32 a and 33 a. This enables the second laser oscillator30 b to oscillate in response to lights having the preset wavelengths.

The optical amplifiers 31 a and 31 b are preferably semiconductoroptical amplifiers for a 1550 nm band used for optical communications.The optical pulses P10 to be quantized are inputted into the opticalthreshold filter 25 have a 1552.52 nm band. The optical filters 32 a and33 a of the first laser oscillator 30 a may be constituted by FBGs whosecenter wavelength is 1549.32 nm. The optical filters 32 b and 33 b ofthe second laser oscillator 30 b may be constituted by FBGs whose centerwavelength is 1558.98 nm. Alternatively, different wavelength bands maybe used.

The first and second laser oscillators 30 a and 30 b are joined by anoptical coupler 34, so that laser outputs produced by one laseroscillator are inputted into the other laser oscillator. In thisexample, 60% of lights which are outputted from the optical amplifier 31a of the first laser oscillator 30 a is inputted into the optical filter32 a while 40% of the lights is inputted into the optical amplifier 31 bof the second laser oscillator 30 b. Alternatively, the splitting ratioof the optical coupler 34 may be 50/50.

An optical coupler 35 is placed between the optical amplifier 31 b andthe optical filter 33 b. The output pulse P25 is outputted via theoptical coupler 35. In this example, the optical coupler 35 has thesplitting ratio of 50/50. Different splitting ratios may be used.

The first laser oscillator 30 a has an output intensity for rippingcarriers off from the optical amplifier 31 b of the second laseroscillator 30 b, thereby stopping the second laser oscillator 30 b. Onthe contrary, the second laser oscillator 30 b does not have an outputintensity for stopping the first laser oscillator 30 a. Therefore, onlythe first laser oscillator 30 a is designed to oscillate at the time ofdefault.

The following describes how the optical synthesizer 24 periodically andsequentially transmits the optical pulses P23 a to P23 e to the opticalthreshold filter 25. The optical synthesizer 24 outputs the first pulseP23 a, which is received by the optical filter 32 a of the first laseroscillator 30 a. As described above, the optical filter 32 a is designedto reflect lights whose wavelength is slightly different from thewavelengths of the optical pulses P10 to be quantized (i.e., thewavelength of the optical pulses P23 a to P23 e). Therefore, the opticalfilter 32 a passes the optical pulse P23 a from the optical synthesizer24. The optical pulse P23 a is inputted into the optical amplifier 31 avia the optical coupler 34.

Since there is a slight difference between a reflective wavelengthcausing the laser oscillation and the wavelength of the optical pulseP23 a, the optical pulse P23 a rips carriers off from the opticalamplifier 31 a, which lowers the laser output of the first laseroscillator 30 a. If the laser output of the first laser oscillator 30 ais below the minimum value for completely suppressing the operation ofthe second laser oscillator 30 b, the second laser oscillator 30 bstarts laser oscillation. Even if the laser oscillation by the secondlaser oscillator 30 b is slight, a slight laser output is sent into theoptical amplifier 31 a, which further weakens a laser output of thefirst laser oscillator 30 a. The weaker the laser output of the firstlaser oscillator 30 a, the stronger the laser output of the second laseroscillator 30 b. The lowering of the laser oscillation in the firstlaser oscillator 30 a and the increase of the laser output of the secondlaser oscillator 30 b are repeated, so that the flip-flop state of thefirst and second laser oscillators 30 a and 30 b is instantly switchedover at an accelerated speed. The laser output of the second laseroscillator 30 b is transmitted as the output pulse P25 (the quantizedoptical pulse P12) via the optical coupler 35.

In the optical threshold filter 25, the laser output of the first laseroscillator 30 a is reduced in accordance with the light intensity of theoptical pulse P23 a from the optical synthesizer 24. When the foregoinglaser output is below a minimum light intensity necessary for completelysuppressing the oscillation of the second laser oscillator 30 b, thestates of the first and second laser oscillators 30 a and 30 b areswitched over. Thereafter, the second laser oscillator 30 b startsoscillation, and produces the output pulse P25. As described above, theoutput pulse P25 is produced by the oscillation of the second laseroscillator, this is effective in signal regeneration and assuring a highcontrast.

In the flip-flop constituted by the first and second laser oscillators30 a and 30 b, when the second laser oscillator 30 b is in operation,the first laser oscillator 30 a remains inactive. The first laseroscillator 30 a remains completely inactive under the followingconditions. The laser output of the second laser oscillator 30 b in thedefault state is very weak compared to the laser output of the firstlaser oscillator 30 a in the default state. In this state, it isimpossible to completely suppress the oscillation of the first laseroscillator 30 a. In short, when the second laser oscillator 30 b isoscillating, the first laser oscillator 30 a is prevented fromoscillating because of a sum of the laser output of the second laseroscillator 30 b and the optical pulse arriving from an external unit(the optical synthesizer 24). In this case, the optical pulse P23 aarrives at the first laser oscillator 30 a from the optical synthesizer24, so that an amount of lights incident on the first laser oscillator30 a is increased. If the laser output of the first laser oscillator 30a is slightly below the minimum necessary light quantity for suppressingthe laser output of the second laser oscillator 30 b, the oscillatingstate of the first or second laser oscillator 30 a or 30 b is changed.In this state, when the sum of the light quantity of the optical pulseincident on the first laser oscillator 30 a and the laser output of thesecond laser oscillator 30 b becomes slightly below the minimum lightamount for completely suppressing the laser output of the second laseroscillator 30 b, the oscillation state of the first or second laseroscillator 30 a or 30 b is switched over. During the change of theoscillation state, when the sum of the light amount of the optical pulseincident onto the first laser oscillator 30 a and the laser output ofthe second laser oscillator 30 b in the default state is a necessaryminimum light amount for completely suppressing the oscillation of thefirst laser oscillator 30 a.

When the first optical pulse P23 a arrives at the optical thresholdfilter 25 from the optical synthesizer 24, the flip-flop state ischanged. While the second laser oscillator 30 b transmits the outputpulse P25, the quantity of light arriving at the optical thresholdfilter 25 is reduced at the trailing edge of the first optical pulse P23a. If the sum of the light quantity of the optical pulse P23 a arrivingat the first laser oscillator 30 a and the laser output of the secondlaser oscillator 30 b in the default state is below the foregoingminimum light quantity, the first laser oscillator 30 a startsoscillating even if the second laser oscillator 30 b is oscillating.This means that the flip-flop state of the first and second laseroscillators 30 a and 30 b is changed.

In the optical threshold filter 25, the first laser oscillator 30 a hasthe output intensity for ripping the carriers off from the opticalamplifier 31 b of the second laser oscillator 30 b in order to stop theoscillation of the second laser oscillator 30 b. On the contrary, thesecond laser oscillator 30 b does not have such an output intensity.Therefore, the optical pulse is inputted into the first laser oscillator30 a, which enables the switch-over of the state of the flip-flop of thefirst and second laser oscillators 30 a and 30 b.

As described above, the optical threshold filter 25 is designed tosequentially receive the optical pulses P23 a to P23 e from the opticalsynthesizer 24. The optical pulses P23 a to P23 e have the lightintensities depending upon the light intensities of the optical pulsesP10 to be quantized. Specifically, the first optical pulse P23 a has thelargest light intensity, and the remaining optical pulses P23 b to P23 ehave the light intensities which are gradually reduced. It is assumedthat the threshold value P_(th) denotes the intensity of external lights(the optical pulses) for the first laser oscillator 30 a to send thelaser output to the second laser oscillator 30 b via the optical coupler35 and to completely suppress the oscillation of the second laseroscillator 30 b. The flip-flop of the optical threshold filter 25 isoperated in response to any one of the optical pulses P23 a to P23 ewhich has the light intensity above the threshold value P_(th), so thatthe output pulses P25 will be outputted. Refer to FIG. 4. On thecontrary, when an optical pulse whose light intensity is below thethreshold value P_(th), the flip-flop remains unchanged (i.e., thesecond laser oscillator 30 b does not oscillate), so that no outputpulse P25 will be transmitted. The optical quantizing unit 12 canproduce the number of output pulses P25 (quantized optical pulses P12)in accordance with the light intensity of each optical pulse P10 (P10 a,P10 b, P10 c, . . . ) to be quantized.

The quantized optical pulses P12 from the optical quantizing unit 12 aresent to the optical binary counter 13 (shown in FIG. 1). The opticalbinary counter 13 includes an assortment of optical logics, and performsthe binary conversion of the quantized optical pulses P12 into encodedoptical pulses P13. The encoded optical pulses P13 are outputted as3-bit pulses as shown in FIG. 2D.

The optical sampling unit 11 of the optical A/D converter 10 receivesthe analog optical signals S10 (see FIG. 2A), sequentially samples themwith preset clock frequencies, and produces the optical pulses P10 to bequantized (see FIG. 2B). The optical pulses P10 to be quantized (i.e.,the optical pulses P10 a to P10 e) have signal levels corresponding tothe light intensities at preset sampling points a to e. The followingdescribe how the five optical pulses P10 a to P10 e to be quantized aretransmitted.

The optical pulses P10 a to P10 e to be quantized are sampled by theoptical sampling unit 11, and are sequentially transmitted to theoptical quantizing unit 12, which quantizes the received optical pulsesP10 a to P10 e, and produces quantized optical pulses P12 (see FIG. 2C).The number of the quantized optical pulses P12 (P12 a to P12 e)corresponds to the light intensities of the each optical pulses P10 a toP10 e to be quantized. The quantized optical pulses P12 a to P12 e aretransmitted to the optical binary counter 13, which performs the binaryconversion of the received optical pulses P12, and produces codedoptical pulses P13 (see FIG. 2D). The coded optical pulses P13 (i.e.,P13 a to P13 e) are assigned binary 3-bit codes denoting lightintensities 1 to 4. A 3-bit code 010 denotes the intensity 2, and isassigned to the optical pulse P13 a which is coded on the basis of thequantized optical pulse P12 a. The 3-bit 001 denotes the intensity 1,and is assigned to the optical pulse P13 b which is coded on the basisof the quantized optical pulse P12 b. A 3-bit code 011 denotes theintensity 3, and is assigned to the optical pulse P13 c which is codedon the basis of the quantized optical pulse P12 c. A 3-bit code 100denotes the intensity 4, and is assigned to the optical pulse P13 dwhich is coded on the basis of the quantized optical pulse P12 d. A3-bit code 011 denotes the intensity 3, and is assigned to the opticalpulse P13 e which is coded on the basis of the quantized optical pulseP12 e. The analog optical analog signals S10 are converted into thepulses P13 a, P13 b, P13 c, P13 d and P13 e, and are outputted insuccession by the optical A/D converter 10.

As described above, the optical divider 21 sequentially divides theoptical pulses P10 (P10 a, P10 b, P10 c, . . . ); the divided opticalpulses P10 are transmitted to the optical filters 23 a to 23 e via thedelay lines 22 a to 22 e; a plurality of optical pulses P23 a to P23 ehave light intensities which are gradually reduced when one of theoptical pulses (e.g., P10 a) is assumed to have a maximum lightintensity; the optical filters 23 a to 23 e pass the divided opticalpulses P10 with the different transmittances to produce the opticalpulses P23 a to P23 e having the foregoing light intensities; and theoptical synthesizer 24 periodically and sequentially transmits theoptical pulses P23 a to P23 e to the optical threshold filter 25 atdifferent timings.

Only when receiving one of the optical pulses P23 a to P23 e which havethe light intensity above the threshold value P_(th), the opticalthreshold filter 25 sends the output pulses P25 (i.e., quantized opticalpulses P12). The number of quantized optical pulses P12 depends upon thelight intensity of the optical pulses P10 to be quantized.

According to the invention, the optical pulses P10 i to P10 ₅ to bequantized are divided on the basis of the number of quantizing levels,and are transmitted to the optical threshold filter 25 via the opticalfilters 23 a to 23 e having the different transmittance. This enablesone optical threshold filter 25 to quantize the optical pulses, which iseffective in simplifying the optical quantizing unit 12 and the opticalA/D converter 10 including the optical quantizing unit 12.

Other Embodiments

In the first embodiment of the optical quantizing unit 12, the opticalfilter array 23 is placed downstream of the delay line array 22.Alternatively, the optical filter array 23 may be placed upstream of thedelay line array 22.

Further, the laser oscillator 30 a (or 30 b) includes the opticalamplifier 31 a (or 31 b) which is placed between the optical filters 32a and 33 a (or 32 b and 33 b). Alternatively, the laser oscillators maybe of a circular type or a micro-ring type that does not need anyoptical filters, and so on.

Still further, the optical threshold filter 25 may be replaced with anoptical threshold filter 50 including a bistable semiconductor laser 51as shown in FIG. 6. The bistable semiconductor laser 51 includes asaturable absorber 53 sandwiched between gain areas 52. By setting avalue of a current applied to the lasers, the bistable semiconductorlaser 51 oscillates lasers only when intensities of lights arriving froman external unit are above a preset value. The use of the opticalthreshold filter 50 is effective in further simplifying the opticalquantizing unit 12.

An optical threshold filter 60 constituted by a nonlinear etalon may beused in place of the optical threshold filter 25, as shown in FIG. 7.With the optical threshold filter 60, a nonlinear media 61 (e.g., madeof GaAs/GaAlAs superlattice film) is placed between multi-layer mirrors62 in order to constitute the resonator. This structure enables thetransmittance of the optical threshold filter 60 to have very strongnon-linear characteristics in response to an increase of intensity ofincident light.

Referring to FIG. 8, the optical quantizing unit 12 may be integratedusing a waveguide. In FIG. 8, a depiction of the optical thresholdfilter 25 is abbreviated, and a light path extends between the opticaldivider 21 and the optical synthesizer 24 is depicted. Further, thenumber of quantizing levels is 8. The optical pulse P10 to be quantizedis inputted into the optical divider 21. The optical divider 21 may be aY-branch type or a gap type coupler using evanescent wave coupling, andso on. By adjusting a splitting ratio of the coupler, the opticaldivider 21 also serves as the light transmitting filter array 23. Thedivided optical pulses P10 to be quantized are transmitted via a curvedlight path having inner and outer path, so that each one of the opticalpulses P23 is output at different delay time. The optical pulses P10 aretransmitted to the optical synthesizer 24 via the delay line array 22,so that a string of optical pulses P23 to be inputted to the opticalthreshold filter 25 will be produced. In the embodiment shown in FIG. 8,it is assumed that the optical divider 21 also functions as the opticalfilter array 23, and each light path has the same coupling ratio at theoptical synthesizer 24. Alternatively, the functions of the opticalfilter array 23 may be carried out by the optical divider 21 and theoptical synthesizer 24, and so on. Alternatively, different arrangementof light path may be used. The foregoing light integrating circuit maybe realized using the optical lithographic process. This is effective indownsizing the light path between the input section and the opticalthreshold filter 25.

1.-10. (canceled)
 11. A method for quantizing optical pulses,comprising: dividing first optical pulses to be quantized, in accordancewith a number of quantizing levels; sending each of the divided firstoptical pulses to each of a plurality of delay lines, so as to delay thedivided first optical pulses in the delay lines by different delaytimes; transmitting each of the divided first optical pulses, which havebeen delayed in the delay lines, to each of a plurality of opticalfilters passing the divided first optical pulses with differenttransmittances; synthesizing each of the divided first optical pulseswhich have passed through the respective optical filters; andtransmitting a string of quantized second optical pulses when lightintensities of the first optical pulses are above a preset thresholdvalue.
 12. The method of claim 11, wherein the second optical pulses aretransmitted by an optical threshold filter including laser oscillators.13. The method of claim 11, wherein each of the second optical pulses istransmitted by an optical threshold filter including a first laseroscillator whose laser output is reduced in response to the firstoptical pulses which arrive from the optical filters, and a second laseroscillator which receives the laser output from the first laseroscillator, and starts oscillation in response to the reduction of thelaser output from the first laser oscillator, the optical thresholdfilter transmitting a laser output from the second laser oscillator. 14.The method of claim 13, wherein the first and second laser oscillatorsrespectively include semiconductor optical amplifiers.
 15. The method ofclaim 11, wherein the second optical pulses are transmitted by anoptical threshold filter including a bistable semiconductor laser havinga saturable absorber.
 16. The method of claim 11, wherein the secondoptical pulses are transmitted by an optical threshold filter includinga resonator constituted by a nonlinear media sandwiched by multilayermirrors.